The Christmas special saw the return of Danny Pink, Clara Oswald's boyfriend and former Coal Hill School maths teacher — albeit only in Clara's dream state. It also dealt with Clara finally moving on from her grief.

On a frosty night before Christmas, Clara Oswald awakens to the sound of an object crashing on her rooftop. Putting on her dressing gown, she leaves her room to investigate and discovers Santa Claus, his elves Ian and Wolf, reindeer and sleigh upon her roof, having crashed after an accident. Ian quickly informs Santa that they've been sighted. They weakly attempt to pass themselves off as ordinary people, but after Clara sees the reindeer flying loose in the sky, they reveal their true identities. One elf points out the beard Santa grew to hide his identity has been public for some time.

Clara tries to deny his existence as a fairytale, having always believed her parents had given her presents instead of him, which the elves laugh at. Santa even lists items Clara wanted at Christmas; everything Clara wanted was actually practical items, instead of other items for hobbies and such. Santa questions if she still believes in fairy-tales, as the TARDIS materialises. The elves are surprised to have been upstaged; it's not very often that happens. The Doctor emerges, telling a stunned Clara that he's back; he then orders her to remain absolutely silent and go into the TARDIS right away. She obeys.

The Doctor leans in close to Santa and states that he knows what is going on, and what is at stake. Santa questions if he truly does, and tells him that before the night is through, the Doctor will be glad for his help. The Doctor departs by wishing Santa a "Happy Easter", and tells him that no-one likes his tangerines. The elves laugh, having previously said the same thing to Santa, but weren't believed.

Inside the TARDIS, Clara comments on how much she had missed it, as the Doctor takes off. He tells Clara that she should trust nothing and be critical towards everything, concluding that the most important question is if she truly believes in Santa Claus. She answers that right now, she does (believing Father Christmas brought the Doctor back as a gift).

At a base in the North Pole, a group of scientists are tracking Shona as she enters an infirmary. One of the scientists called Ashley tells Shona not to think about "them" and to concentrate on something else. She enters the infirmary, and begins dancing to Slade's "Merry Xmas Everybody" which appears to take her mind off the victims. She reaches the end of the infirmary, by which point the Doctor and Clara appear. Shona initially mistakes them for ghosts, calling the Doctor a skeleton man.

As Clara wonders what's happened to the victims, they wake up and begin moving. Shona warns them not to think about what they can see (mumbling the song to herself), and the Doctor deduces that the creatures attached to the victim's faces are both deaf and blind, but they use telepathy to keep a constant image of themselves in someone's memory, allowing them to 'see'. In an attempt to save Clara, the Doctor insults Danny, succeeding in flooding her mind with emotion. As the victims close in, the scientists appear with guns ready to attack, and then several crab-like creatures appear from the ceiling and attack the group.

Santa arrives to save the day

Suddenly a wall blasts off and an army of toys march in. Everyone looks out the hole to see Santa riding Rudolph, and dismounting. The reindeer is humourously calmed by Santa, using a car fob. Santa demands an answer as to what is happening; he doesn't want any trouble on his property. To everyone's surprise, Santa is able to order the victims back to bed. Everyone heads into a different room to avoid waking them again. Shona wonders who Santa is, to which the Doctor sarcastically states to take a guess, listing the Easter Bunny and the Tooth Fairy. Santa tells the Doctor that an invasion has begun. Clara wonders if it's an invasion of elves, angering them (as it's racist); they retort that she's one to talk as she's almost as short as them.

Santa presents a captured creature, which the Doctor recognises as a Kantrofarri or "dream crab". Santa's existence is questioned by everyone (especially Shona, whom he angers), but he encourages them to focus on the dream crabs as Earth may have just seen its last Christmas depending on how many are there. The Doctor encourages Ashley to deduce what they are and what they do; he then explains that they are dying and most likely asleep. He points to the TARDIS, explaining that reality and fantasy are difficult to tell apart because both are ridiculous. He and Clara then admit to each other that they lied - he about finding Gallifrey, she about Danny returning from the Nethersphere - so the other could move on with their lives to be happy.

The Doctor asks Ashley to show him CCTV footage of the other crew members arriving on the base. The footage shows the four crew members discovering the creatures, buried in the snow. The footage then shows one of the creatures coming down from the ceiling, which sends the camera static. The Doctor explains that when a dream crab attacks someone, it places them into a telepathically induced dream state. Whilst the victim is "dreaming", the crab drills into the victim's head and eats away at their brain. And if you are thinking about a dream crab, one is most likely coming for you. Albert compares the creatures to facehuggers, confusing the Doctor; after Albert explains that he means the horror movie Alien, a miffed Doctor explains that it's really offensive and must be why real aliens keep invading Earth.

Clara goes to check on the captured crab, only to find it missing from the container. It begins to stalk her, at which point Clara tries thinking of maths equations and then of Danny to stop it. Unfortunately, it breaks through the table she's hiding under, putting her into a dream state. Clara finds herself at her house on Christmas day with Danny; when Clara is confused by his outfit, Danny jokes he's "Father Easter". The Doctor and the group arrive at Clara's side, where the Doctor tries to work out a way to remove the crab without killing Clara. Ashley states the only way to remove the crab would be to kill Clara. He refuses to do so, asking Santa (who can't be targeted by the crabs) to bring one to place on his head.

The Doctor meets with the dream Danny, who greets him in military fashion to annoy him just like the real one would. Clara knows it's not real, but wanted to spend more time with Danny; however, in a few minutes the dream crab will have burrowed too deep into her head to be removed. The dream Danny explains that while he did die, it was to save Clara; the rest of the world got lucky. He encourages Clara to move on, but to mourn him for five minutes a day. The Doctor and Clara awaken, causing the crabs to fall off and disintegrate; "Carnivore's hazard; food has teeth, too."

Despite waking up, Clara complains of a pain on the right side of her head. Shona compares it to the "ice cream pain" and confirms she has the same pain. The Doctor then works out that they are in fact still in a dream, having been attacked when they arrived. With some help from Santa — who is revealed to be a manifestation of everyone's brains telling them something is wrong — the group are able to wake up. They escape the infirmary, barely evading the victims. The Doctor bids farewell to the group as Clara goes after him; the Doctor explains that unless the group is stupid enough to let the dream crabs out, they should be fine. He also explains he can't keep constant watch, stating that there are other dangerous things on the planet.

Clara notes they both saw Santa on her rooftop, meaning they're still dreaming. They return to the base where the Doctor questions the scientists on why they only have four base manuals for a crew of eight people. They then come to the realisation that the victims are really themselves, or rather dream constructs of what's coming to kill them. The victims awaken and transport themselves through the CCTV, killing Albert in the process. The group travel outside and try to work out a way to escape, of which the Doctor suggests using the TARDIS to escape. Clara notes it isn't the real TARDIS, and as they head towards the TARDIS, dream constructs of themselves appear. Worse, the constructs have multiplied; "logic of a nightmare" the Doctor notes.

Clara doesn't want to wake up

The Doctor tells everybody to use their imagination to get themselves home, and so they all dream of Santa, who appears in his sleigh and rescues the group. Having been rescued by Santa, the memories of the group's real lives start coming back to them, meaning that they're waking up in the real world. Shona suggests that they exchange phone numbers so they can meet up once they awaken, but the Doctor states that it's unlikely that they will remember anything that's happened in the dream. One by one, everyone except Clara awakens, who wants to stay in the dream world forever.

The Doctor travels to Clara's house and is successful in removing the crab from her face with his sonic screwdriver, but finds 62 years have passed for her since they said goodbye in the coffee shop after Danny's death; this places Clara at age 89. In Clara's living room, the Doctor brings in Christmas hats, believing that anything seems funny if the hats are worn. They discuss her life since they last met. She travelled, but never married despite numerous admirers; she does admit that there was one other man besides Danny she liked "but it wouldn't work out; he was impossible." They pull a cracker together, with the Doctor helping the elderly Clara do so. He apologises for not returning for her sooner, wishing that he had. From the doorway, Santa again appears and asks if he really does wish for that. They are surprised this is yet another crab-induced dream. Santa whispers "Wakey-wakey!!"

The Doctor awakens for real this time and he travels to Clara's house, removing the crab from her face again to reveal that she is still young; humorously, the Doctor can't tell whether Clara is young or not. Realising what could have been, the Doctor invites Clara to resume her travels in the TARDIS with him. She happily agrees and he whisks her away, still in her nightie. As they dematerialise, a tangerine sits on the window sill and there is a faint hint of sleigh bells...

Not every person who worked on this adventure was credited. The absence of a credit for a position doesn't necessarily mean the job wasn't required. The information above is based solely on observations of the actual end credits of the episodes as broadcast, and does not relay information from IMDB or other sources.

Santa is visibly offended when both his elves and later the Doctor tell him that no one likes tangerines.

Shona dances to the song "Merry Xmas Everybody" by Slade to distract herself from thinking about the dream crabs while she is in the infirmary on the North Pole base.

Professor Albert compares the dream crabs to the facehugger version of the title creature from the 1979 sci-fi/horror movie Alien. The Doctor has heard of neither the facehuggers nor the film, and is visibly appalled that a horror movie is named Alien - "No wonder everyone keeps invading you!", he exclaims.

The Doctor tells Clara not to get too attached to the people at the arctic base because it "isn't Facebook."

When the Doctor leaves Santa on the roof-top, he mockingly tells Santa, "Happy Easter".

When Ashley asks who Santa is, the Doctor ask her who she thinks he is and gives the "Tooth Fairy" and "Easter Bunny" as possible examples.

In Shona's Christmas Day itinerary, the films Alien, The Thing from Another World and Miracle on 34th Street are named, along with "Thrones". The first three films are very clearly sources for the shared dream state (the facehuggers, a group of scientists trapped in an isolated Arctic base by a deadly alien creature, and a man who believes he is Santa, respectively).

This episode saw a one-off modification to the title sequence. The clock faces the TARDIS flies through are blue and icy, and the TARDIS is covered in snow, which both dissipate. The names of the lead actors dissolve into snowy particles, and the subsequent time vortex has flying snowflakes.

For the first time for this title sequence, more than two people are credited. Peter Capaldi and Jenna Coleman's names appear normally, then Nick Frost's name appears between the Doctor Who logo and the episode title credit.

This is the second episode in a row to feature a notable modification, the first being Death in Heaven, where Jenna Coleman was credited before Peter Capaldi and her eyes were shown instead of his. This was done in order to lend credence to Clara's claim in the pre-credits sequence that she was the Doctor in order to fend off an attacking Cyberman.

Steven Moffat, Jenna Coleman and Peter Capaldi later confirmed that Coleman was initially going to leave the series with this episode, but last-minute lobbying by Capaldi and Moffat convinced her to stay on for one more series. According to Moffat, the decision was made rather late in the process, perhaps as late as the readthrough, resulting in an eleventh hour revision to the ending.

To keep the appearance of Danny Pink a surprise, Samuel Anderson was uncredited in Radio Times and on the BBC website. This was his final appearance as a regular.

When the Doctor accidentally antagonises Clara about what Danny Pink may be up to, she slaps the right hand side of his face, yet he clutches the left side.

Shona's hair changes style and length throughout the episode, even within the space of a few minutes. The production team intended for her changing hair to be a clue to the dream, but sometimes her hair changes within the same 'dream'.

The CGI shots of Big Ben are extraordinarily out of scale, portraying Big Ben as tiny or the sleigh and it’s occupants as massive.

The Series 8 finale (and possibly more of the season) was a dream. This is due to the Doctor waking up on what appears to be the same volcanic planet seen within Clara's induced dream state in TV: Dark Water, coupled with the fantastical afterlife-related plotline of the two-parter and the fact the Doctor's dream is under way by the time Santa appears in the mid-credits sequence of TV: Death in Heaven.

Clara and the Doctor are still dreaming as the episode ends. This is left ambiguous due to the presence of one of Santa's tangerines at the end. However it should be noted that Santa Claus does exist within the televised Whoniverse per TV: A Christmas Carol. As a time jump occurs between this episode and the start of Series 9, the Doctor and Clara may have resolved the situation off-screen.

This is also the second time in consecutive storylines in which Clara experiences a realistic dream state that is forced upon her; the previous one also involved the death of Danny. (TV: Dark Water)

Shona and Santa both liken the Doctor to a magician because of his appearance. He previously noted this about himself (TV: Time Heist), as did Clara (TV: The Caretaker). Oddly, they make this observation despite the fact he is actually wearing his hoodie costume variant, and in fact beginning with this episode he adopts more casual outfits and is not seen again in anything approaching "magician"-like until TV: Face the Raven.

Both Clara and the Doctor reveal to each other that they previously lied to each other on their last meeting to stop the other from worrying: (TV: Death in Heaven)

Clara lets it slip that Danny Pink is still dead, something she had withheld from the Doctor. Danny appears in a crab-induced dream later, in a Santa suit and slightly taller.

A nightie-clad Clara leaves in the dead of night to restart her travels in the TARDIS. Amy Pond's travels with the Doctor began in similar fashion. (TV: The Eleventh Hour)

Like Clara throughout the dream, Amy wore a nightie (without a dressing-gown on top, though) in her first trip in the TARDIS. (TV: The Beast Below)

The Doctor has Clara concentrate on doing sums to distract her from the crabs, something she later does again when trying to avoid the escaped crab in the lab. The Third Doctor used the same technique - albeit with simpler sums - to force Jo Grant to focus on something except the brainstorm being experienced by Axos. (TV: The Claws of Axos)

When the Doctor gains access to Clara's dream and meets Danny, the Doctor greets him "Merry Christmas, PE." He had previously mockingly called Danny "PE". (TV: The Caretaker et.al.)

Danny in return calls the Doctor "sir". He had previously done that to annoy him. (TV: The Caretaker et.al.)

Santa mocks the Doctor, saying he would explain the complicated situation by telling them "it's dreamy-weamy"; similarly, Shona refers to Santa as "beardy-weirdy". The Doctor used (and occasionally borrowed) some expressions like "timey-wimey" (Blink et.al.), "spacey-wacey" (The Doctor's Wife) and "humany-wumany" (The Doctor, the Widow and the Wardrobe).

When the Doctor questions how Santa can have all those presents in his bag, Santa claims that it's "bigger on the inside", to the Doctor's annoyance. (TV: An Unearthly Child, Doctor Who, Rose et.al.)

Clara hugs the Doctor, which initially makes him uncomfortable. The Doctor previously told her that this self doesn't like hugging. (TV: Deep Breath, Listen, Death in Heaven) He doesn't seem to like joining hands, either, but will make an exception for Clara.

Clara kisses the Doctor on the cheek for the first time on screen in this incarnation, and for the first time since she kissed the Eleventh Doctor in TV: The Day of the Doctor.

In Clara's Christmas day-dream, while standing in the hallway, she becomes suspicious about something not being right and sees blackboards all over the hallway telling her that she's dying. Previously the Doctor used chalk and blackboards to write notes in his TARDIS. (TV: Deep Breath, Into the Dalek, Robot of Sherwood, Listen)

Santa was able to tell the Dream-infected people to get back in their beds, telling them it was Christmas Eve and they should go early to bed. The Ninth Doctor was previously able to tell all the Empty-people to go to bed by acting as their requested "Mummy" and grounding them. (TV: The Doctor Dances)

During the final dream state, the elderly Clara tells the Doctor that she learned to fly a plane in the years since her last meeting with him. During their first meeting, the Eleventh Doctor asked her if she could fly a plane, and Clara told him that she couldn't. (TV:The Bells of Saint John)

Contrary to common belief, season 10 kicked off in the last week of December 1972 — not in 1973, as would be expected. Season 10 actually began nine years after season 1 started. In fact, The Three Doctors began nine years to the week after The Daleks first aired.

For the purposes of this list, "Series 4" is considered to be the production series 4, which ran all the way from Time Crash to The End of Time.

The years seen in this section may seem decidedly "off". Remember, however, that this list only gives the first year in which an episode from a series was broadcast. David Tennant, unusual amongst other Doctors, began and ended on special episodes, not regular ones. Thus, his series actually begin in 2005, 2006 and 2007 — not 2006, 2007 and 2008 as is commonly thought.